Does Stainless Steel Rust or Tarnishand Why?
Does Stainless Steel Rust?
In generalstainless steel does not rustbut if it is used or maintained improperlyor in a harsh environmentit will rust or tarnishfor examplethe photo above shows a stainless steel shackle on a cruise ship corrodes due to long-term exposure to the marine environmentthe surface appears brown rust and spots.
The reason why stainless steel does not rust is because it contains chromium (Cr) and nickel (Ni). These two metal elements will form a dense oxide film with oxygenthis layer of oxide is also called a passivation filmwhich enhances the corrosion resistance. When the surface of stainless steel is scratchedit will instantly form a new oxide filmthus preventing it from further rusting and corrosion.
Why Does Stainless Steel Rust or Tarnish?
Stainless steel rust because the oxide film on its surface is continuously destroyedtherefore loses its rust proof effectresulting in the continuous oxidation of iron.
What Causes Stainless Steel to Rust?
Halides are one of the most serious factors that cause stainless steel to rustthey can destroy the passivation film. Halides include chloridefluoridebromideiodide and some inter-halogen compounds. Among themchloride is the most common in nature and daily life. For examplechlorine ions (Cl-) are widely present in saltsaucephysiological salinebleachdisinfectantsalt water lakeand sea wateretc. If stainless steel is polluted by these factors for a long time and is not cleaned in timeit will rust or tarnish.
Other factors that can cause stainless steel to rust include: long-term exposure to inorganic acids such as nitric acidsulfuric acidhydrochloric acidhydrofluoric acidetc. Organic acids such as acetic acidformic acidetc. Alkaline such as sodium hydroxide (NaOH)etc.
Different Grades of Stainless Steel Have Different Corrosion Resistance
Different grades of stainless steel have different rust resistance due to different Cr-Ni content. Other chemical elements that affect corrosion resistance are: contents of carbon (C)phosphorus (P)sulfur (S)molybdenum (Mo)titanium (Ti)copper (Cu)niobium (Nb)etc.
According to the organization methodsthey can be divided into 4 classifications:
- Austenite: 200 (essentially Cr-Mn alloys) and 300 series (essentially Cr-Ni alloys)for example201302304 (18/8 stainless steel)310316317321etc.
- Ferrite: essentially Fe-Cr alloys400 seriessuch as 430
- Martensite: essentially Fe-Cr-C alloys400 seriessuch as 410420440etc.
- Duplex stainless steel: including 220422052507etc.
In terms of chloride corrosion resistancethe rust resistance ranking of common stainless steel grades is as follows:
904L > 2205 > 316L > 316 > 304L > 304 > 302 > 201 > 430 > 440.